1. Field of the Invention
The present invention relates to a backlight unit for a flat panel display and a flat panel display apparatus having the same and, more particularly, to a backlight unit for a flat panel display with a uniform brightness gradient across the entire light-emitting surface and a flat panel display apparatus having the same.
2. Description of the Related Art
Unlike self-emissive flat panel displays, non-emissive flat panel displays, such as liquid crystal display (LCD) panels, need external light to produce an image. Thus, a backlight unit is located behind a non-emissive flat panel display and illuminates light on the flat panel display such as an LCD panel in order to produce an image. The backlight unit for the flat panel display is used as a backlight unit for an LCD device or a surface light source system such as an illuminating sign.
Backlight units are classified, as either direct light type backlight units or edge light type backlight units according to the position in which a light source is arranged. A point light source having an approximately point-shaped light-emitting portion or a linear light source having a linear light-emitting portion disposed along one direction may be used as a light source for an edge light type backlight unit. Representative examples of the linear light source and point light source are a cold cathode fluorescent lamp (CCFL) having two electrodes at opposite ends within a tube and a light emitting diode (LED) (or laser diode), respectively.
Korean Laid-open Patent Publication No. 2003-4021 discloses a backlight unit for a flat panel display using a planar hologram filed by an applicant of the present invention. FIG. 1 is a schematic cross-sectional view of the disclosed backlight unit. Referring to FIG. 1, the backlight unit includes a light source 51 disposed within a housing 55, a light guide panel (LGP) 20 for guiding light emitted by the light source 51 by the use of total reflection, a reflective member 31 that is disposed below the LGP 20 and upwardly reflects light escaping from the LGP 20, and a transmissive diffusion sheet 11 that is disposed above the LGP 20 and widely diffuses light escaping upward from the LGP 20. The LGP 20 has a holographic pattern 25 repeated continuously with a grating period p′ at a top surface thereof. The light incident on the LGP 20 is totally internally reflected into the LGP 20 by top and bottom surfaces thereof and propagates along the LGP 20. Some of light directed onto the holographic pattern 25 is diffracted upward by the holographic pattern 25 and escapes approximately perpendicular to the LGP 20. A light beam guided by the LGP 20 escapes sequentially as the distance from the light source 51 increases so the amount of light beam propagating along the LGP 20 decreases away from the light source 51. That is, a large amount of light is incident onto a holographic grating close to the light source 51 but the amount of light directed onto the holographic grating progressively decreases away from the light source 51.
In a conventional backlight unit, the holographic pattern 25 is formed in a regular pattern that is continuously repeated over the entire surface of the LGP 20 with a grating period p′ and a depth d′ and has a fixed pattern regardless of the distance from the light source 51. Thus, diffraction efficiency defined as the ratio of the amount of light diffracted by the holographic pattern 25 to the amount of light directed onto the holographic pattern 25 is maintained constant over the entire light-emitting surface of the LGP 20 and the amount of light escaping through the LGP 20 decreases as the distance from the light source 51 increases.
FIG. 2 illustrates the profiles of a brightness distribution for a conventional backlight unit for a flat panel display. Each profile shows brightness data for five measurement positions in an LGP partitioned according to the distance from a light source. The measurement positions have no units because the measurement positions 1 through 5 can be understood as points spaced apart by the same distance. Thus, “1” may be the point located at the front end of the LGP, and “5” may be the point located at the rear end of the LGP. The LGP has a fixed pattern to maintain diffraction efficiency constant. More specifically, the profiles a, b, c and d respectively show brightness distributions when LGPs having fixed diffraction efficiencies of 3%, 6%, 9%, and 11% are applied. As evident from FIG. 2, in all cases, as the distance from the light source increases, brightness level significantly decreases. Brightness uniformity which is defined as the percentage of minimum to maximum brightness level is only about 58%. There is much room for improvement. Because emission brightness obtained from the backlight unit has a large variation depending on measurement position, a flat panel display apparatus employing the backlight unit suffers a variation in brightness level depending on position on a display surface and a blemish in brightness, thus providing degraded image quality and display performance.